When a user subscribes to a telecomm carrier's telephony services, the subscribing user can utilize a user equipment (UE), such as a mobile handset, to make calls. A carrier that provides access to a Long Term Evolution (LTE) network can provide such telephony services using Voice over LTE (VoLTE). VoLTE is a fairly reliable technology, but the UE may, in some instances, be unsuccessful in establishing a VoLTE call. This can be due to various issues that arise from time-to-time inhibiting establishment of a VoLTE session. For example, a network outage may occur during a particular call attempt, and the UE may not receive a network response that is needed to establish a dedicated evolved packet system (EPS) bearer for setting up a VoLTE call. Such a network outage may be due to a failure at a mobility management entity (MME) that is responsible for sending the response to the UE.
Rather than wait an indefinite period of time for such an issue to resolve itself from the network side, the UE is typically configured to utilize one or more timers during call setup to proactively resolve the issue from the client side. These timers are configured to timeout if something that is expected to occur during call setup does not occur within a set period of time. Accordingly, after a call setup timer is triggered, the UE can wait a set period of time until the timer expires, and thereafter take some remedial action. The action taken by the UE can include, among other possible actions, dropping the call so that the user is not left waiting indefinitely for an answer, or reattempting the call using a different approach (e.g., a different call path) in an effort to establish the call for the user.
Existing call setup timers implemented in today's UEs are problematic because the call setup timers can, in some instances, cause downstream network issues, like network outages, due to the way the timers are designed. A salient example is in the context of emergency calls. When a user dials 911 (in the United States) on his/her UE, the call is routed to an appropriate (e.g., nearest) public safety answering point (PSAP). Local regulations for PSAPs may differ, but most PSAPs are required to take extra steps to avoid a situation where an emergency call goes unanswered. For instance, some PSAPs are required to record every call attempt to the PSAP, some record every call that begins ringing, some require a person to answer the call, and/or, if an Interactive Voice Response (IVR) system is in place, a call can be answered by the IVR system, and some PSAPs call back every call that is not answered by a person or an IVR system. Imagine a scenario where multiple UEs are frequently reattempting calls to a single PSAP, but failing to establish the call on each reattempt due to a network issue. In this scenario, the frequent expiration of call setup timers on the individual UEs are causing the frequent reattempts, and the PSAP may be unable to recover from the issue due to the sheer volume of incoming calls. As a consequence, the problem may “snowball” out of control to a point where the PSAP can no longer handle the high volume of traffic, and calls may not get through to the PSAP. As a result, one or more of these calls may go unanswered and emergency response services may be compromised. In recognizing that the aforementioned problem is, at least partly, caused by poorly-designed call setup timers, there is a need for further improvements to UE call setup procedures utilizing timers.